1. Field of the Invention
The present invention relates to a fuel control apparatus having an intake air quantity sensor used for controlling fuel supplied to an internal combustion engine. More particularly, it relates to a fuel control apparatus which permits a correction in response to a change with time in a hot wire type intake air quantity sensor.
2. Discussion of Background
The characteristic of the hot wire type intake air quantity sensor is changed by materials deposited on the surface of a hot wire, whereby there is caused an error in an amount of fuel supplied to the engine. As a result, the problems of deterioration of exhaust gas and reduction in operational performance are invited.
There is the same problem in a vane type intake air quantity sensor due to a change of characteristic depending on materials deposited on its sliding part. The change of characteristic caused by the deposite affects a flow rate of intake air passing around the intake air quantity sensor. Correction of the change of characteristic may be made by a negative feed-back control by using an air-fuel ratio sensor. However, there is a case that the correction of the change of characteristic has to be made in a region where it is impossible to employ the negative feed-back control. For such specific case, there is known a method of learning and correction as shown in, for instance, Japanese Unexamined Pat. Publication No. 150057/l983. This publication discloses an apparatus for correcting an air-fuel ratio by negative-feeding-back of the output of an air-fuel ratio sensor provided in the exhaust pipe of an engine, wherein a value of negative feed-back is stored in a memory so that a basic value for fuel control is corrected in the region other than a negative feed-back region on the basis of data stored in the memory.
In a case that a learning and correction method is carried out in a region (where a high flow rate is formed and an air-fuel ratio thicker than the theoretical air-fuel ratio is required) other than a negative feed-back region by estimating a value of correction at the region which allows correction of characteristic change of an intake air quantity sensor by the negative feed-back control, when there is caused temporarily an error in an air-fuel ratio peculiar to that region at the time of the correction by the negative feed-back control, a value obtained by the learning and correction by estimation becomes incorrect and it enhances an air-fuel ratio error in the high flow rate region. As such temporarily error, there is an air-fuel ratio error produced when vaporized full caught in a canister is purged into the intake air passage of the engine. The effect by purging the fuel is large when the engine is driven with a low load, i.e. when an amount of intake air is small. On the other hand, the effect is small when an amount of intake air is large. Accordingly, it is impossible to use the method of learning and correction by estimation unless the effect by the purging is removed.